1. Field of the Invention
The present invention is directed to a method of manufacturing a photomask comprising phase shifters.
2. Description of the Prior Art
FIGS. 1A and 1B are a plan view and a partial cross sectional view of a conventional photomask 100. In FIGS. 1 and 1B, a transparent substrate 1 is coated with a plurality of light shielding layers 2 each including a light shielding pattern 2p of a predetermined dimension. The light shielding layers 2 are formed on the transparent substrate 1 such that a line-space grating is periodically constituted. Each alternate space between adjacent light shielding layers 2 is filled with a phase shifter 3.
FIG. 2 is a schematic diagram of an exposure device. The exposure device includes a light source 200 for emitting ultraviolet light. Ultraviolet light from the light source 200 impinges on the photomask 100 through a lens 300. The ultraviolet light entering the photomask 100 is partially transmitted through uncoated regions (space regions) while partially blocked by the light shielding layers (line regions) 2. Transmitted light impinges on a wafer 500 through a lens 400.
Now, operation of the photomask 100 will be explained. First, the photomask 100 is mounted to the exposure device of FIG. 2 and the light source 200 is turned on so that ultraviolet light from the light source 200 impinges on the photomask 100 through the lens 300. Since the photomask 100 merely allows partial transmission, the ultraviolet light is present in some area but is not present in other area immediately below the photomask 100. More particularly, it is only in the uncoated regions of the photomask 100 not provided with the patterns 2p where the ultraviolet light is allowed. The phase shifter 3 causes light passing through the same to be phase shifted by 180.degree.. Hence, light passed through both the transparent substrate 1 and the phase shifter 3 is 180.degree. out of phase with light passed only through the transparent substrate 1.
The transmitted light is focused on the wafer 500 by the lens 400. In exposure-regions of the wafer 500 corresponding to the uncoated regions (space regions), the light passed only through the transparent substrate 1 is affected by the light passed through the phase shifter 3. Thus, the light intensity at the centers of the exposure-regions is increased, so that the half-value width is decreased. As a result, images corresponding to the patterns 2b are printed with high resolution on the wafer 500.
FIG. 3 is a plan view of patterns printed on the wafer 500 in the above manner. It is readily seen that the printed patterns are in desirable condition in regard with X direction. Due to the phase shifter 3 formed along X direction in every other uncoated region (space region), the patterns 2p have good contrast. In regard with Y direction, however, the printed patterns are in an undesirable condition because adjacent patterns 2b are bridged by a bridge 2b. FIG. 4 explains the reason why. In a boundary area 3a, where the edge of the phase shifter 3 meets the transparent substrate 1 with respect to the Y direction, the phases of the abutting regions are opposite. Hence, light intensity is reduced almost to zero on the wafer 500 in a region corresponding to the boundary area 3a.
As heretofore described, pattern printing using the conventional photomask 100 creates a bridge connecting adjacent patterns. Thus, truly desired pattern cannot be obtained by using the conventional photomask 100.